In the fabrication of composite articles, it is typical to assemble a plurality of orientated dry fiber layers and to shape them to match the shape of the article. The assembly, known as a "preform", is placed in a mold and resin is injected into the mold to infiltrate the fiber layers. The resin is cured to produce the article. This is known as a "resin transfer molding" (RTM) process.
When assembling the various fiber layers, or "plies", it is common to encounter various junctions made between a pair of plies which results in a void space or gap. For example, with reference to FIG. 1a, a structural member 1 known as a sinewave spar has an upper flange 2 supported by a sinusoidally shaped web 3. FIG. 1b shows a cross-section of such a structure which includes a plurality of first plies 4 and a plurality of second plies 5 which form the web 3 of the structure with a third plurality of plies 6 placed over a top surface of the first and second plies, to form the flange 2. When producing such a structure, a problem develops in that a gap 7 is formed where the plies meet.
If the preform having these assembled plies is placed into a mold, and resin injected into the mold, the various fiber layers may be forced to enter the gap, thus distorting the fiber orientation, as illustrated by the phantom lines. This produces a weakness in the structure. This fiber distortion does not completely fill the gap, and a resin rich area results which is a site for initiating cracks and delaminations. Such a part would be rejected, due to the potential for failure. Thus, it is important to maintain the fiber plies in the proper curved shape while at the same time reducing the resin rich region formed in the gap between the plies.
Various methods were considered for solving this problem. One utilized a cured insert made of chopped fibers or another similar discontinuous structural material which is shaped, cured and then located in the gap before the cap plies 6 are added. While such an insert provides support for the curved portion of the layers, it does not adequately bond with the adjacent plies since it fails to incorporate any resin during injection. This leaves a discontinuity in the interface between the insert and plies, and thus, a weakness in the structure remains. In addition, the discontinuous fiber is very fragile when pressed and cured into discrete lengths, making it difficult to handle during preform assembly.
Another method utilized a pre-impregnated unidirectional fiber ("prepreg") tape, that is folded and pressed into a shaped tool and debulked. The shaped material is then removed from the tool and placed into the preform gap. It was found that the pre-impregnation of the fibers failed to allow sufficient resin infiltration during injection for full integration with the part. Again, a discontinuity remained between the prepreg and the adjacent plies.
Another alternative utilized a dry fiber braided rope 8, as shown in FIG. 1c. However, the braid, having interlocked fibers, maintained its circular cross-section and failed to properly fill the gap. Thus, it continued to allow fiber distortion and resin rich regions.
To address these problems, a filler is needed for the gaps between plies that is capable of being handled by an assembler without fiber shifting. The time required to produce the assembled preform must also be minimized. The filler, once located in the preform assembly, must be near net shape to avoid distorting the adjacent plies and be inspectable for defects before resin injection so that any defects can be corrected before the final part is produced. Utilizing materials which must conform to the gap during tool closure do not provide adequate assurance that the part will be successfully produced. When inspection is only available after part construction, there is a significant potential for producing rejected parts, wasting labor, material and mold time.